The de-ubiquitinase USP33 has been shown to possess either tumour-promoting or inhibitory effect on human cancer cells. However, all these findings are mainly based on in vitro cell culture models, and the in vivo evidence, which is more plausible to digest the functional role of USP33 in carcinogenic process, is still lacking. Here, we demonstrate that USP33 modulates DNA damage responses including cell cycle arrest and apoptosis induction through associating with p53. It directly interacts with p53 to mediate its de-ubiquitination and further  stabilisation under DNA damage condition. Depletion of USP33 induces an enhanced level of p53 ubiquitination, which de-stabilises p53 protein leading to impaired DNA damage responses. Furthermore, USP33 silencing shows either promoted or inhibited effect on cell proliferation in human cancer cells with p53 WT and mutant background, respectively. Consistently, mice with hepatocyte-specific USP33 knockout are more sensitive to nitrosodiethylamine (DEN)-induced hepatocarcinogenesis compared to wild type mice. Thus, our in vitro and in vivo evidences illustrate that USP33 possesses anti-tumour activity via regulating p53 stability and activity.